Republicans lead by 51% to 41% among registered voters in Gallup weekly tracking of 2010 congressional voting preferences. The 10-percentage-point lead is the GOP’s largest so far this year and is its largest in Gallup’s history of tracking the midterm generic ballot for Congress.

To put this in perspective, until this month, the biggest lead the GOP had held in the history of Gallup’s polling was … five points. Why the eeyorism, then? Well, (a) Rasmussen has new generic ballot numbers out today too and the GOP’s actually lost a few points since last week, driving them down to their smallest lead since mid-July. Not sure how to square that with Gallup, especially since Ras polls likely voters and Gallup polls registered voters. The enthusiasm gap should mean a bigger spread among the former than the latter (and until today, it has), and if Gallup’s numbers are merely a reaction to last week’s dismal economic news, it’s surpassingly strange that the same reaction isn’t showing up in Rasmussen. Also, (b) Gallup’s generic-ballot polling has already produced one freaky outlier this summer. Granted, today’s numbers are more credible because they’re part of a trend, but read this Jay Cost piece about how bouncy Gallup’s numbers have historically been at times. Hmmmm.

This Gallup result is so large, I had to see what it shows in the Swingometer. As always, I boil it down to two party results. In 2008 we had a 56 D – 44 R split, and this Gallup simplifies to a 45 D – 55 R split. So the swing is from a D+12 to an R+10, or a 22 point swing.

So right now, that means Gallup of all polls, using Registered Voters, is projecting in the Swingometer a 60 seat Republican gain for a 238 R-197 D majority. The last time an election took the Democrats that low was the election of 1946, saith Wikipedia. Election night in 2004 took them to 202 for the second lowest.

Rasmussen today, by contrast, shows only a 20 point swing, a 57 seat Republican gain, and a 235 R – 200 D majority, still lower than an election since Truman has taken the House Democrats. If I then take the mean of these two and double weight the Rasmussen Likely Voter poll, I get R+58, the new projection.

The “enthusiasm gap” is even more pronounced. Gallup finds that Republicans are now twice as likely as Democrats to be “very” enthusiastic about voting come November, the largest such advantage of the year.

I’m obliged to add that anything can happen during the next two months. But more than any old thing will be required if the Democrats are to avoid a crushing defeat at the end of those two months.

The biggest problem for the Democrats is that there seem to be very few things that can happen between now and Election Day that can reverse the Republican momentum. The latest round of economic reports seem to establish fairly clearly that the economy is likely to remain flat or depressed during that time period and I doubt we’ll be getting any good news out of the jobs report that will be released this coming Friday, and it is primarily the economy that is driving voter anger at this point in time. Outside of some massive scandal that hurts Republicans or an international crisis that causes the public to rally around the President, both of which are unlikely, the pattern we’re in now is likely to be the one we’re in on Election Day. That’s bad news if you’re a Democrat.